In optical systems involving the generation and controlled radiation of long or continuous pulses of light, such as spectroscopy, or solar simulation, where high intensity, color correct illumination of sensitive working areas is required, such as in fiber optics illumination devices, it is advantageous to have a light source capable of producing the highest possible light flux density. Products utilized in such applications include short arc inert gas lamps. An existing short arc lamp includes a sealed quartz chamber containing a gas pressurized to several atmospheres, and an opposed anode and cathode defining an arc gap. A window provides for the transmission of the generated light, and a reflector may be positioned surrounding the arc gap.
Various applications require small short arc lamps, such as in video projectors and medical and dental equipments. Sub-miniature arc lamps are produced to meet the needs of these applications. In an existing design of a sub-miniature arc lamp, an anode and a cathode are mounted inside a quartz tube with a top and a base. The anode and the cathode are separated by a short arc gap. The joint between the quartz tube and the top and the joint between the tube and the base are sealed. The quartz tube is filled with inert gas. During operation, the breakdown voltage is exceeded across the short arc gap between the anode and the cathode, an illuminating flow of electrons is discharged from the cathode to the anode.
Generally speaking, there are four major reasons for lamp failure, including electrode erosion, contamination of the fill gas, cracked glass to metal seals, and explosion caused by devitrification or cracking of the quartz tube. Erosion of the electrodes causes a reduction in light output and, potentially, failure of the quartz tube. Devitrification of the quartz tube, caused by the high temperature inside the quartz tube during operation, is the removal or destruction of the glassy quality of the quartz tube. In addition to devitrification, the high temperature inside the quartz tube can also lead to the cracking of the quartz tube. Eventually, the devitrification and cracking of the quartz tube will lead to breakage of the quartz tube. Besides damaging the lamp, breakage of the quartz tube can cause user injuries as well.
Moreover, high peak currents discharged through the lamp during operation generate instantaneous high temperature on the inner wall of the quartz tube. The high temperature on the inner wall of the quartz tube causes the silicon oxide in the quartz tube to reduce to silicon and oxygen, which causes contamination of the fill gas. In addition to high temperature, devitrification will also lead to oxygen generation from the quartz tube. The electronegative nature of the oxygen inhibits the electron flow and effectively raises the breakdown voltage of the lamp. An increased breakdown voltage impedes ignition and triggers reliability problems with the lamp.
A prior solution to reduce the contamination inside the quartz tube is to use gas additives to reduce tungsten wall coverage inside the quartz tube. However, the gas additives also make processing the sub-miniature arc lamps at high temperature difficult.
Another prior solution is to operate the lamp in a vertical position to minimize devitrification of the quartz tube. Horizontal operation in high pressure quartz lamps tends to cause early failures due to tube devitrification problems. However, having to operate the arc lamp in vertical orientation complicates the design of the optical equipment using the arc lamp.